Developers used in electrophotographic imaging apparatuses such as printers and copy machines may be classified into one-component developers consisting exclusively of toner and two-component developers including toner and a magnetic carrier. In a system using a one-component or two-component developer (hereinafter, referred to as one-component development system or two-component development system), toner is charged with a constant amount of charges and then supplied to a photoreceptor having an electrostatic latent image thereon to form a toner image. This toner image is then transferred onto paper by a transfer member, and fixed thereto as a final image by heat and pressure.
A two-component development system may include a mixture of toner and a magnetic carrier in a ratio in a developing unit. In the two-component development system, the toner may be charged by friction with the magnetic carrier. For a long lifespan of such two-component development systems, it is important that the initial characteristics of the toner and the magnetic carrier can be maintained for the long period of frictional charging till the end stage of the lifespan, since it is crucial to maintain the toner charge amount constant during an electrophotographic process in terms of image concentration and quality control. In general, the longer the period of use of toner, the smaller the toner charge amount. This is due to state change of the toner and magnetic carrier for the following expected causes.
A) Change in Toner
The main cause of change in toner charge amount may be a behavior of the external additive. The friction between toner particles and the magnetic carrier by stirring for a long time may cause the external additive on the toner surface to become buried inside the toner particles. The external additive may also be separated from the toner particles by friction or shear force. Such a change in the state or amount of the external additive may cause change in the toner charge amount.
B) Change in Magnetic Carrier
Friction between the magnetic carrier and the toner particles for a long time may reduce the thickness of a resin layer coated on a surface of the magnetic carrier or may cause separation of the resin layer from the surface of the magnetic carrier. When the external additive separated from the toner particles adhere to the surface of the magnetic carrier, the toner may not be effectively charged. Furthermore, an organic substance such as a releasing agent from the surface of the toner particles may contaminate the surface of the magnetic carrier.
Therefore, to maintain the toner charge amount constant, it is important to design an effective external additive and to reduce surface contamination of the magnetic carrier.
JP 2008-170489 discloses a toner for developing electrostatic latent images with good fixing characteristics, fluidity, and durability, wherein, in differential scanning calorimetric (DSC) thermograms of two types of waxes (wax A and wax B) which are a component of the toner particle, the onset temperatures O(A) and O(B) of wax A and wax B and the endothermic peak temperatures P(A) and P(B) thereof satisfy the following conditions: O(A)<O(B) and P(B)≤P(A).
However, the control of such parameters is not enough to provide a two-component developer that may effectively inhibit contamination of the magnetic carrier even in a printing process performed for a long time and consequentially suppress image defects caused by a reduced charge amount of the toner.